From long experience on the space shuttle and various space stations,
we have some knowledge of how mammals, especially people, respond
to 0-g. We have even more experience with 1-g on Earth. But we still
don't know what happens in between.

What,
for example, will happen to humans on Mars where the surface gravity
is 0.38-g? Is that enough to keep human explorers functioning properly?
And, importantly, how easily will they readapt to 1-g, once they return
to Earth?

A team of scientists and students from the Massachusetts Institute
of Technology (MIT), the University of Washington, and the University
of Queensland, in Australia, plans to explore these questions. They're
going to do it by launching mice into orbit.

"What we're doing," explains Paul
Wooster, of MIT, and program manager of the Mars Gravity Biosatellite
project "is developing a spacecraft that is going to spin to
create artificial gravity." The satellite will spin at the rate
of about 34 times each minute, which will generate 0.38-g -- the same
as gravity on Mars.

The team hopes to launch the Biosatellite in 2006. The mice will
be exposed to Mars-gravity for about five weeks. Then, says Wooster,
they'll return to Earth alive and well. The mice will descend by parachute
and land near Woomera, Australia, inside a small capsule reminiscent
of NASA's old Apollo capsules.

The Biosatellite project is the first investigation conducted at
this gravity level, says Wooster. Financed in part by NASA, the project
is also unique "due to the heavy involvement of students in all
aspects of the work, including planning the science, designing the
spacecraft, raising the funds, and managing the overall effort,"
he adds.

The research will focus on bone loss, changes in bone structure,
on muscle atrophy, and on changes in the inner ear, which affects balance.
"The main thing we're trying to do," says Wooster, "is
to chart a data-point between zero-gravity and one-gravity."

As they orbit the earth, the mice, each in its own tiny habitat,
will be painstakingly observed. Each habitat will have a camera, so
that the researchers can monitor mouse activity. Each will have its
own pump-driven water supply, so that each mouse's water consumption
can be tracked.

Each
mouse's wastes will be collected in a compartment beneath its habitat;
the compartment will contain a urinalysis system checking for biomarkers
that indicate bone loss.

Each habitat will also be equipped with a body mass sensor, which
will take frequent readings. This will also allow the researchers
to track how the weight of the mice changes over the course of the
five weeks.

Each mouse will also have toys to keep it busy. "We may give
them a wooden block to chew on," says Wooster. That'll keep them
happy, and will also prevent them from chewing on the habitat. They
might have a small tube to run through.

No wheels, though, says Wooster, because NASA has learned that exercise
can counteract some of the effects of low-gravity on astronauts. A
mouse with a wheel in its cage can actually run several miles a day.
"We don't want to give the mice a countermeasure in terms of
exercise."

The students will be using only female mice, says Wooster. That's
partly because female mice eat slightly less than male mice, decreasing
the mass that must leave Earth. But more importantly, some studies
suggest that females are affected more strongly by lowered gravity
than the males.

Those studies, though, weren't conducted in true partial gravity.
Rather, they were done by suspending the hind legs of the animals,
so that the mice are only able to feel part of their weight on the
ground. The simulated Mars gravity inside the Biosatellite will be
much more realistic.

Right:
Much of the Mars Gravity Biosatellite is still on the drawing board.
Shown here is a cutaway design diagram of a mouse habitat for the
spacecraft. Credit: MarsGravity.org

Through the three participating universities, more than 250 students
have been involved in the Biosatellite project. The project is being
led and coordinated by MIT, which is also managing the animal habitats
and life support systems. The University of Washington is in charge
of providing electrical power, propulsion, attitude control, thermal
control, and all the communications to the ground. The University
of Queensland is in charge of the entry, descent, and landing systems,
including the heat shields and parachutes.

"I think that one of the big contributions of the Biosatellite,"
says Wooster, "is the educational benefit for the students involved."
So many people, he says, have been inspired by this project, and have
learned from it. "Plus we're going to be getting back information
that nobody's ever had before, data that have been missing in the
planning of human missions to Mars."

How might humans respond to gravity on Mars?

With the successful landing of NASA's rover Spirit, that question
seems closer and closer to one we'll need to solve.